1. Field of the Invention
The present invention relates to a ceramic junction member, a ceramic heater and a gas sensor.
2. Description of the Related Art
Conventionally, in a gas sensor employing a solid electrolytic member such as an oxide sensor, a ceramic heater is disposed in the gas sensor so as to heat the solid electrolytic member. A widely employed ceramic heater has been a type in which a heating resistor formed of a metal such as tungsten or molybdenum is embedded within a ceramic base member such as alumina. Further, an electrode pad electrically connected to the heating resistor is provided on the outer surface of the ceramic base member. A metallic coupling terminal for applying an external voltage to the heating resistor is soldered to the electrode pad.
In order to join different kinds of materials such as the ceramic base member, the electrode pad and the solder material, a technique has also been proposed which employs an electrode pad having two layers, namely, a first layer and a second layer (see Patent Document 1). The first layer contains a relatively large amount of ceramic material which is the main component of the ceramic base member and the second layer contains a relatively small amount of the ceramic material and in which the component of the soldering material for joining the electrode pad with the coupling terminal is sintered.
[Patent Document]
[Patent Document 1] JP-A-2007-22908
3. Problems to be Solved by the Invention
In the case of forming the second layer on the first layer, the second layer may be shifted from the first layer when the size of the first layer is substantially same as that of the second layer (that is, when the second layer is formed so as to deviate from the contour of the first layer). In this case, the misaligned portion of the second layer is not provided on the first layer but directly on the ceramic base member. Thus, adhesion of the second layer with respect to the ceramic base member as compared with that of the first layer is degraded. This is because the second layer is specified so as to adhere to the first layer and the soldering material, such that the misaligned portion of the second layer may exfoliate from the ceramic base member. Further, the entirety of the second layer may exfoliate starting at the misaligned portion of the second layer. That is, the coupling terminal may become separated from the ceramic base member.